Apparatus and system for welding preforms and associated method

ABSTRACT

An assembly for positioning a structural assembly for friction stir welding, and a system and method for friction stir welding the structural assembly are provided. The assembly includes a frame defining an aperture therein, and at least one structural member positioned within the aperture. The assembly also includes at least one spacer positioned within the aperture such that the spacer and structural member substantially fill the aperture to secure the structural member within the frame, as well as a substrate secured to the frame and positioned adjacent to the structural member in an overlapping configuration to define an interface between the substrate and structural member. In this configuration, the substrate and structural member are capable of being friction stir welded together.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to friction stir welding and, moreparticularly, to positioning a structural assembly for friction stirwelding and friction stir welding the structural assembly to form apreform.

2) Description of Related Art

Friction stir welding is a process in which a rotating tool, such as apin or probe, is urged into and/or through a workpiece, e.g., to joinmultiple members of the workpiece in a solid state or to repair cracksin a workpiece. Typically, the pin extends from a shoulder, which can beflat, concave, or otherwise contoured, and the shoulder is urged againstthe workpiece so that the pin is urged therein. The pin is then urgedthrough the workpiece to form a continuous weld joint. For example,during one conventional friction stir welding process, the probe of therotating tool is plunged into a workpiece or between two workpieces by afriction stir welding machine to produce the required resistance forceto generate sufficient frictional heating to form a region ofplasticized material. The tool can be tilted approximately 3° relativeto the workpiece such that the trailing edge of the shoulder is thrustinto and consolidates the plasticized material. Upon solidification ofthe plasticized material, the members of the workpiece are joined alongthe weld joint. The magnitude of force exerted by the friction stirwelding tool must be maintained above a prescribed minimum in order togenerate the required frictional heating. Friction stir welding isfurther described in U.S. Pat. No. 5,460,317 to Thomas et al., thecontents of which are incorporated herein by reference.

Friction stir welding has been demonstrated to be a successful joiningmethod for forming certain types of joints, such as the butt joint wherethe probe is inserted within abutting edges of two structural members,or a lap joint in which the probe is inserted in a directionsubstantially perpendicular to the interface between overlappingstructural members. In addition, other types of friction welding havealso been shown to be useful for forming certain joints.

Methods have also been used to friction stir weld preforms. One methodincludes inserting the probe through a structural member to one side ofa center web substrate. Structural members are then added to the opposedsecond side of the substrate by turning the part over and inserting theprobe through the additional structural members and into the substrateto weld the entire structure together. A second method involves weldingthe structural member to the substrate by inserting the probe throughthe substrate and into the underlying structural member. A secondstructural member is stacked on top of the substrate (opposite thepreviously welded structural member), and the probe is inserted throughthe second structural member and into the substrate to weld thestructure together. Both methods require that each structural member beseparately clamped for welding, which is expensive and time consuming.

It would therefore be advantageous to provide a system for friction stirwelding preforms that is relatively inexpensive and effective. It wouldalso be advantageous to provide an assembly to secure structures havingvarious sizes during friction stir welding of preforms. It would furtherbe advantageous to provide an assembly that promotes efficient assemblyand disassembly before and after friction stir welding the structuresinto a preform.

BRIEF SUMMARY OF THE INVENTION

The invention addresses the above needs and achieves other advantages byproviding an assembly for positioning a structural assembly for frictionstir welding preforms, as well as an associated system and method forfriction stir welding preforms. The structural assembly is capable ofbeing arranged in a variety of configurations to secure the structuralmembers during friction stir welding, to create various preforms, and topromote efficient assembly before friction stir welding and disassemblyafter welding.

In one embodiment of the present invention an assembly for positioning astructural assembly for friction stir welding is provided. The assemblyincludes a frame defining an aperture therein, and at least onestructural member positioned within the aperture. The assembly alsoincludes at least one spacer positioned within the aperture such thatthe spacer and structural member substantially fill the aperture tosecure the structural member within the frame, as well as a substratesecured to the frame and positioned adjacent to the structural member inan overlapping configuration to define an interface between thesubstrate and structural member. In this configuration, the substrateand structural member are capable of being friction stir weldedtogether.

In various embodiments of the present invention, the frame defines firstand second opposed edges, the aperture extending between the first andsecond edges to define a thickness, and wherein the frame is the samethickness as the structural member, while the frame and structuralmember may be thicker than the spacer. In addition, the spacer may bepositioned within the aperture to define a gap between at least aportion of the spacer and the structural member, or the spacer couldcontact at least a portion of the structural member. Further, the spacermay include a variety of cross sections, such as rectangular, circular,elliptical, or trapezoidal. The assembly may include a pair of spacersthat are in abutting engagement along an angular surface relative to thestructural member. Also, the assembly may include a pair of spacers inabutting engagement along an angular surface relative to the substrate,where one of the pair of spacers has a greater thickness than arespective other of the pair of spacers. The substrate may be secured tothe frame with fasteners, or with a vacuum drawn through a port definedin the frame. The substrate could be positioned adjacent to thestructural member such that at least one of a lap and butt joint may beformed. Furthermore, the assembly may also include at least onestructural member and spacer positioned within the aperture on anopposite surface of the substrate in an overlapping configuration todefine an interface between the opposite surface of the substrate andthe structural member.

In an additional embodiment of the present invention, a system forfriction stir welding is provided. The system includes a frame definingan aperture therein, and at least one structural member positionedwithin the aperture. The system also includes at least one spacerpositioned within the aperture such that the spacer and structuralmember substantially fill the aperture to secure the structural memberwithin the frame. In addition, a substrate is secured to the frame andpositioned adjacent to the structural member in an overlappingconfiguration to define an interface between the substrate andstructural member, and a probe is operable to reciprocate and penetratethe interface to friction stir weld the substrate and structural membertogether. In a variation of the probe, the probe extends substantiallyperpendicular to the interface and may include a shield extendingsubstantially parallel to the interface. The shield comprises anon-rotating support member positioned adjacent to the structural memberand spacer that is capable of securing the spacer within the apertureduring friction stir welding.

The present invention also provides a method for friction stir welding.The method includes providing a frame defining an aperture therein, andpositioning at least one structural member and at least one spacerwithin the aperture such that the spacer and structural membersubstantially fill the aperture to secure the structural member withinthe frame. The method also includes securing a substrate to the frameand adjacent to the structural member in an overlapping configuration todefine an interface between the substrate and structural member, andurging a reciprocating probe through the interface to friction stir weldthe substrate and structural member together into a preform.

In additional aspects of the method, the method includes positioning thespacer within the aperture such that a gap is defined between at least aportion of the spacer and the structural member, or such that the spacercontacts at least a portion of the structural member. The method mayinclude forming one of a lap and butt joint between the substrate andthe structural member. The method may include the additional step ofremoving the spacer from the aperture following the step of insertingthe reciprocating probe. The probe is typically urged substantiallyperpendicular to the interface. The method may further includepositioning a shield carried by the probe adjacent to the structuralmember and spacer, where the shield is capable of securing the spacerwithin the aperture during the urging of the probe. The method may alsoadvantageously include positioning at least one structural member andspacer on an opposite surface of the substrate and thereafter urging thereciprocating probe through the interface defined between the oppositesurface of the substrate and the at least one structural member suchthat a structural member is capable of being friction stir welded to theopposite surface of the substrate. In addition, the method may includemachining the preform to form a structural assembly having apredetermined configuration.

In a variation of the present invention, an additional assembly forpositioning a structural assembly for friction stir welding is provided.The assembly includes a frame defining an aperture therein, and aplurality of structural members positioned within the aperture. Theassembly also includes a plurality of spacers positioned within theaperture such that the spacers and structural members substantially fillthe aperture to secure the structural members within the frame. In thisconfiguration, the structural members are capable of being friction stirwelded to form a butt joint. In a further aspect of the assembly, asubstrate is secured to the frame and positioned adjacent to thestructural members in an overlapping configuration to define aninterface between the substrate and structural members, wherein thesubstrate and structural members are capable of being friction stirwelded together.

The present invention therefore provides an assembly that is capable ofarranging structural members in a variety of configurations prior tofriction stir welding. The assembly eliminates separate tooling forclamping each of the structural members prior to friction stir welding,which consequently reduces the cost of friction stir welding preforms.The spacers are capable of adequately securing the structural membersduring friction stir welding, and are shaped and sized to promoteremoval of the spacers following welding even when the structuralmembers expand. Furthermore, the assembly is capable of positioning thestructural members for fabricating a variety of preforms that may laterbe machined into a finished part.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a plan view of an assembly for positioning a structuralassembly for friction stir welding preforms according to one embodimentof the present invention;

FIG. 2 is a cross-sectional view of the assembly of FIG. 1;

FIG. 3 is an elevation view of a spacer shown in FIG. 1 according to oneembodiment of the present invention;

FIG. 4 is a plan view of an assembly for positioning a structuralassembly for friction stir welding preforms according to anotherembodiment of the present invention;

FIG. 5 is a cross-sectional view of the assembly of FIG. 4;

FIG. 6 is a plan view of an assembly for positioning a structuralassembly for friction stir welding preforms according to yet anotherembodiment of the present invention;

FIG. 7 is a cross-sectional view of the assembly shown in FIG. 6;

FIG. 8 is a cross-sectional view of the assembly shown in FIG. 4,illustrating a probe shield according to one embodiment of the presentinvention; and

FIG. 9 is a flowchart illustrating the method for friction stir weldingpreforms according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the invention are shown. Indeed, this invention may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

Referring now to the drawings and, in particular to FIGS. 1–2 there isshown a structural assembly 10 that is capable of being friction stirwelded into a preform. The structural assembly 10 includes a frame 12and a plurality of structural members 14 and spacers 16 arranged withinan aperture 18 defined within the frame. A substrate 20 is positionedadjacent to the structural members 14, typically in an underlying oroverlying relationship, such that a probe 22 can be used to frictionstir weld the structural members and substrate together into a preform.A preform is generally an assembly of multiple structural members 14that approximate the desired finished shape of a part, and which theycan be machined or otherwise trimmed to the finished shape. The term“preform” is not meant to be limiting, as there could be any number andconfiguration of structural members 14 that are welded to the substrate20. In addition, the preform could be at least two structural members 14friction stir welded together without including a substrate 20.

Generally, at least two structural members 14 are positioned in anoverlapping configuration with the substrate 20 to define an interface24 therebetween that can be welded to form a joint weld. A lap jointgenerally extends substantially perpendicular to the interface 24,through one of the structural members 14, through the interface, and atleast partially through the substrate 20. Similarly, the lap joint couldextend through the substrate 20 and interface 24 and at least partiallyinto the structural members 14. Alternatively, other types of joints cansimilarly be formed. For example, a butt joint can be formed by abuttingthe edge surfaces of the structural members 14 and welding through aninterface 24 of the abutting surfaces. The structural members 14 canalso be positioned and welded in other configurations, for forming bothlap and butt joints, and any number of structural members 14 can bejoined together or joined to the substrate 20.

The structural members 14 and substrate 20 can be formed of a variety ofmaterials including, but not limited to, aluminum, aluminum alloys,titanium, titanium alloys, steel, and the like. Further, the preform caninclude structural members 14 and substrates 20 of similar or dissimilarmaterials, for example, structural members formed of different alloys ofa base metal, including alloys that are unweldable or uneconomical tojoin by conventional fusion welding techniques. Unweldable materials,when joined by conventional fusion welding techniques, producerelatively weak weld joints that tend to crack during weldsolidification. Such materials include aluminum and some aluminumalloys, particularly AA series 2000 and 7000 alloys. The use of frictionstir welding permits structural members 14 and substrates 20 formed ofunweldable materials to be securely joined. Friction stir welding alsocan be used to securely join weldable materials to other weldable and tounweldable materials. Thus, the materials that form the preform can bechosen from a wider variety of light weight, high strength metals andalloys, thereby facilitating reduction of the overall weight of thepreform and a finished part formed therefrom.

The preform formed according to the methods of the present invention canbe used in a variety of applications, including, for example, frames,panels, skins, airfoils, and the like for aeronautical and aerospacestructures such as aircraft and spacecraft, for marine vehicles,automobiles, and the like, as well as for other applications outside ofthe transportation industry. The friction stir weld joints can be usedfor joining large preforms having a variety of geometries. The preformsshould approximate the desired dimensions and configuration of the finalpart to reduce the machining time required during machining, as well asreduce waste material.

As shown in FIG. 1, a plurality of structural members 14 and spacers 16are arranged within the aperture 18. FIG. 2 demonstrates that asubstrate 20 is positioned to overlie the structural members 14, whichdefines an interface 24. A probe 22 is inserted through the substrate 20and interface 24 into an underlying structural member 14. As shown inFIG. 5, the probe 22 may also be inserted from the opposite directionthrough a structural member 14 and interface 24 into the underlyingsubstrate 20. The probe 22 may be inserted substantially perpendicularto the interface 24, although it is understood that “substantiallyperpendicular” is not meant to be limiting and could include extendingthe probe perpendicular or tilting the probe at a small angle (e.g., 0to 3 degrees) relative to the substrate 20 and structural member 14. Theprobe 22, which is made of a harder material than either the substrate20 or structural member 14, rotates as it is translated to generatefrictional heat within the substrate and structural member to at leastpartially plasticize the substrate and structural member. Upon coolingof the plasticized material 25, the substrate 20 and structural member14 are joined along the resulting weld joint. Friction stir welding isfurther described in U.S. Pat. No. 5,460,317 to Thomas et al., theentire content of which is incorporated herein by reference.

The structural members 14 and spacers 16 are arranged within theaperture 18 defined by the frame 12 such that the structural members aresecured within the frame. The structural members 14 and spacers 16 aresized and shaped to substantially fill the aperture (similar to apuzzle) so that the structural members are secured during friction stirwelding. It is understood that the phrase “substantially fill” is notmeant to be limiting and may be interpreted such that the structuralmembers 14 and spacers 16 are positioned within the aperture 18 suchthat there could be various sized gaps between each of the structuralmembers and spacers, or that there could be no gaps such that thestructural members are firmly fixed within the aperture. Thus, aseparate clamp is not required to secure each structural member 14during friction stir welding. The structural members 14 and spacers 16can be arranged in any desired configuration within the aperture 18, andconsequently, the spacers 16 could be any desired dimension toaccommodate the structural members 14. The structural members 14 shownin FIGS. 1, 4, and 6 are rectangular, although any geometry ofstructural members could be employed with corresponding spacers 16according to additional embodiments of the present invention.

As discussed above, the substrate 20 is positioned adjacent to thestructural members 14 to define an interface 24 therebetween, and it ispossible to secure the substrate to the frame 12 and structural member14 using a variety of techniques. The substrate 20 is typically securedso that there is minimal movement of the substrate during friction stirwelding. The substrate 20 could be secured to the frame 12 withfasteners 26, as shown in FIG. 2. Alternatively, FIG. 5 illustrates thatthe substrate 20 may be secured within a recess 28 defined within theframe 12. Further, in the instance where the substrate 20 is positionedbetween structural members 14, as depicted in FIGS. 6–7, the substratemay be secured to the frame 12 with a combination of a recess 28 andfasteners 26. Thus, the frame 12 may be any suitable thickness andconfiguration to accommodate various assemblies for friction stirwelding. In addition, the substrate 20 could be secured to an underlyingmachine bed with protrusions and/or a recess defined in the machine bed.In an additional embodiment, the substrate 20 is secured to the frame 12and structural members 14 with a vacuum. A groove or port is defined inthe frame 12 of this embodiment, wherein the substrate is positionedadjacent to the frame, and a vacuum is applied through the groove orport to retain the substrate adjacent to the frame 12 and structuralmembers 14 to define the interface 24. Therefore, a variety oftechniques may be used to secure the substrate 20 to the frame 12 andstructural members 14 during friction stir welding, which are efficientto set up and to remove so that adjustments may be made duringprogressing stages of fabricating the preform.

The spacers 16 are generally used to secure the structural members 14within the aperture 18 during friction stir welding. The spacers 16could be any dimension to accommodate any number or dimension ofstructural members 14. For example, FIG. 1 illustrates that the spacers16 could be rectangular, while FIG. 3 shows that there could be a pairof spacers joined along an angular surface defined respective to thesubstrate 20. Furthermore, the spacer 16 could be a pair of spacersjoined along an angular surface defined relative to the structuralmembers 14, as shown in FIG. 4. Using a pair of spacers 16 defined alongan angular surface promotes removal of the spacers following frictionstir welding FIGS. 5 and 7 illustrate that the spacers 16 could becircular or trapezoidal in cross section. The circular and trapezoidalcross sections reduce the amount of area in contact with the structuralmembers 14, substrate 20, and frame 12, which in turn, promotes removalof the spacers 16 following friction stir welding, while also securingthe structural members in a desired configuration within the aperture18. As such, the spacers 16 could be any cross section or dimension sothat the contact area of each of the outer edges of the spacers isreduced when positioned within the aperture 18 and adjacent to thestructural members 14, substrate 20, and frame 12.

Generally, the spacers 16 contact the substrate 20 and frame 12 whenpositioned within the aperture 18, although in additional embodiments ofthe present invention, a gap is defined between the spacers and thestructural members 14 and substrate 20. Because the structural members14 may expand during friction stir welding, providing a gap between thespacers 16 and structural members allows the structural members toexpand slightly without seizing up on the spacers. For example, a gap of0.050 inches may be provided between the structural members 14 andspacers 16. Further, to ensure that the preforms remain withinacceptable dimensional tolerances for subsequent machining when gaps areprovided between the structural members 14 and spacers 16, thestructural members may include excess material on each of its surfacesthat faces the spacers to allow for expansion during friction stirwelding. For instance, in one embodiment of the present invention, thestructural members 14 may include 0.2 inches of excess on each of itssurfaces facing the spacers, although various amounts of excess may beprovided depending on the desired amount of precision or the amount ofmachining required.

Gaps may also be defined between the spacers 16 and the substrate 20.Generally, the frame 12 includes opposed lateral edges, and the aperture18 extends between the lateral edges to define a thickness. For example,one of the pair of spacers 16 shown in FIG. 3 may be substantially thesame thickness (depicted as vertical thickness in this particularembodiment) as the structural members 14, while the corresponding otherof the pair includes a smaller thickness than the structural members.Also, the rectangular spacers 16 shown in FIG. 1 could have a smallerthickness than the structural members 14 and frame 12. Providing asmaller thickness than the structural members 14 and frame 12 defines agap between the spacer 16 and the interface 24, which promotes removalof the spacers following friction stir welding, allows expansion of thesubstrate 20 and structural members 14, and also prevents the spacers 16from bridging, i.e., the spacers do not support the substrate 20 duringfriction stir welding.

The spacers 16 may be of any number of sizes and configurations foraccommodating various structural members 14 and substrates 20. Thus,although FIG. 1 shows three individual spacers 16 and one pair ofspacers, it is possible to have any number, size, and combination ofspacers. Therefore, it is possible to include two, five, or any numberof individual spacers 16, while incorporating spacers having anycombination of circular, trapezoidal, or rectangular cross sections. Thespacers 16 may not only be placed in the aperture 18 between thestructural members 14 and frame 12, as the spacers could be locatedsolely between structural members. Furthermore, there could be no gapsbetween the spacers 16 and the frame 12, structural members 14, andsubstrate 20, or the spacers may be stacked or oriented in variouspositions. Furthermore, different sizes of gaps may be provided betweenthe spacers 16 and the structural members 14 and substrate 20 to allowfor varying amounts of expansion during friction stir welding, whilealso securing the structural members within the aperture 24 and allowingfor removal of the spacers. In addition, the spacers 16 may includevarious cross sections so that the contact area between the spacers andstructural members 16, substrate 20, and frame 14 is reduced. Thus, thespacers 16 could be rectangular or square with rounded edges,elliptical, or even triangular or other shapes in cross section. Thepair of spacers 16 having angular abutting surfaces, such as those shownin FIGS. 3–4 and 7, may abut one another at various angles to promoteremoval following friction stir welding. For example, in additionalembodiments of the present invention, there could be a pair of spacers16 positioned at 45 degrees or even perpendicular to one another.

The orientation and configuration of the spacers 16 promote removalfollowing friction stir welding, as discussed previously. Differenttechniques for removing spacers 16 are used depending on the type ofspacer employed during friction stir welding. For example, the pair ofspacers 16 shown in FIG. 3 would be removed by first removing one of thepair of spacers, such as with a screw or other fastener. The screw couldattach the pair of spacers 16 together, or the screw could be screwedthrough one of the pair until the screw contacts the substrate, andfurther advancing the screw causes the spacer to lift off of thesubstrate. Once one of the pair of spacers 16 is removed, the other ofthe pair can be removed, which in turn, loosens the remaining spacersand resulting preform located within the aperture 18. The remainingspacers 16 may be removed by hand or with fasteners, or the preform maybe removed, leaving the spacers within the aperture 18. In instanceswhere a rectangular, circular, trapezoidal, or other spacer 16 isincorporated, a fastener, such as a screw, could be inserted within thespacer and advanced until the screw contacts the substrate 20 andfurther advancement of the screw causes the spacer to lift off of thesubstrate. It is also understood that the spacers 16 could be removed byhand or by lifting the spacers with tools that lever the spacers out ofthe aperture.

The spacers 16 may be constructed of a variety of materials. The spacers16 are generally made of a material that is capable of withstandingdownward force during friction stir welding, as well as withstandinglateral forces within the aperture while supporting the structuralmembers 14. In addition, the spacers 16 are generally capable ofmaintaining the structural members 14 within a desired dimensionalaccuracy such that the structural preforms can be machined to desireddimensions following friction stir welding. The spacers 16 could beformed by a variety of material processes, such as by sawing individualspacers from a larger block of material, cutting the spacers to adesired size with a water jet, casting, or other suitable process.Furthermore, the spacers 16 may be reusable or expendable, such that aspacer may be used one or more times, or only once for a specificpreform.

As shown in FIG. 8, the probe 22 may include a shield 30. The shield 30does not rotate during friction stir welding, and is thus positionedadjacent to the structural members 14 and spacers 16 such that theshield may slide as the probe is advanced along the structural membersand substrate 20. The shield 30 generally extends laterally outward fromthe probe 22 and provides a sufficient amount of surface area tomaintain alignment with the structural members 14 and substrate 20,while also ensuring that the spacer 16 is adequately secured within theaperture while the probe is plasticizing an area proximate to thespacer. Thus, as shown in FIG. 8, the shield 30 may be positionedadjacent to the structural member 14 and frame 12, while also beingpositioned adjacent to the cylindrical spacer 16. The shield 30 could becircular in cross section and include a slot that allows the probe 22 torotate uninhibited. The shield 30 could be attached to the surroundingmounting of the probe 22 such that the shield does not rotate. It isunderstood that various configurations of shields 30 may be employedwith alternative embodiments of the present invention, while stillproviding sufficient support to the spacers 16 to ensure that thespacers are secured within the aperture and that the probe 22 rotatesunrestrained during friction stir welding.

Different techniques may be used to friction stir weld the structuralmembers 14 and substrate 20 together. For instance, the flowchart shownin FIG. 9 demonstrates a method for friction stir welding preformsaccording to one embodiment of the present invention. The frame 12,defining an aperture 18 therein, is generally secured with fasteners 26to an underlying base to secure the frame. As shown in FIG. 1, thestructural members 14 and spacers 16 are positioned within the aperture18 such that the aperture 24 is substantially filled to secure thestructural members and spacers within the aperture. The substrate 20 isplaced adjacent to the structural members 14 and fastened to the frame12 such that an interface 24 is defined between the substrate and thestructural members. A vacuum could be applied in place of, or incombination with, the fasteners 26 to secure the substrate 20 to theframe 12. The rotating probe 22 is then urged within the substrate,through the interface 24, and into the underlying structural member 14.The probe 22 is translated through the substrate 20 and structuralmember 14 as the probe is rotating to friction stir weld the substrateand structural member together. In this configuration a lap joint, asdescribed above, may be formed between the substrate 20 and each of thestructural members 14. Alternatively, as shown in FIG. 5, the probe 22may be inserted through the structural members 14 and interface 24 andinto the underlying substrate 20 to friction stir weld the structuralmembers and substrate together. In this instance, the substrate 20 maybe secured within a recess 28 defined in the frame 12.

Additionally, FIG. 7 illustrates that a second layer of structuralmembers 14 may be friction stir welded to the opposite side of thesubstrate 20 by first friction stir welding the structural members onone side of the substrate to the substrate and then turning thesubstrate over and then repeating the steps outlined above for FIG. 5 tofriction stir weld the structural members 14 on the opposite side of thesubstrate 20 to the substrate to form the preform. The substrate 20 maybe secured by fasteners 26 to the frame 12, or a recess 28 in the framemay be used in place of, or in combination with, the fasteners.Furthermore, a second layer of structural members 14 may be frictionstir welded to the structural assembly 10 shown in FIG. 2 by stacking asecond layer of structural members 14 on top of the substrate 20 andthen urging the probe 22 through the structural members 14 and interface24 and into the underlying substrate 20.

It is understood that various friction stir welding techniques may beemployed with the present invention in alternative embodiments. Althoughthe techniques described above are directed to forming lap joints, it isunderstood that butt or other joints may also be formed in additionalembodiments of the present invention. For example, the structuralmembers 14 may be friction stir welded together without employing asubstrate 20. In this configuration, the structural members 14 could bejoined with lap or butt joints. In addition, it is understood that thesubstrate 20 could cover only a portion of the aperture 24, such that acombination of lap and butt joints may be formed.

Once the preform is created, a predetermined amount of excess materialcan be machined from the preform to form the machined structuralassembly 10. The machining process can be performed by any known means,including using a manual or computer-guided machining device, such as aCNC machine. Advantageously, because the preforms closely orsubstantially approximate the predetermined dimensions and configurationof the corresponding machined structural assembly 10, the amount ofmachining is relatively small compared to, for example, the amount ofmachining that would be required to machine structural assemblies fromsolid blocks of material.

The present invention therefore provides a structural assembly 10 thatis capable of arranging structural members 14 in a variety ofconfigurations prior to friction stir welding. The structural assembly10 eliminates separate tooling for clamping each of the structuralmembers 14 prior to friction stir welding, which consequently reducesthe cost of friction stir welding preforms. The spacers 16 are capableof adequately securing the structural members during friction stirwelding, and are shaped and sized to promote removal of the spacersfollowing welding even when the structural members 14 expand.Furthermore, the structural assembly 10 is capable of positioning thestructural members 10 for fabricating a variety of preforms that maylater be machined into a finished part.

Many modifications and other embodiments of the invention set forthherein will come to mind to one skilled in the art to which thisinvention pertains having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A method for friction stir welding, comprising: providing a framedefining an aperture therein; positioning at least one structural memberand at least one spacer within the aperture such that the spacer andstructural member substantially fill the aperture to secure thestructural member within the frame; securing a substrate to the frameand adjacent to the structural member in an overlapping configuration todefine an interface between the substrate and structural member; andurging a reciprocating probe through the interface to friction stir weldthe substrate and structural member together to form a preform.
 2. Amethod according to claim 1, wherein positioning comprises positioningthe spacer within the aperture such that a gap is defined between atleast a portion of the spacer and the structural member.
 3. A methodaccording to claim 1, wherein positioning comprises positioning thespacer within the aperture such that the spacer contacts at least aportion of the structural member.
 4. A method according to claim 1,wherein securing comprises forming one of a lap and butt joint betweenthe substrate and the structural member.
 5. A method according to claim1, further comprising removing the spacer from the aperture followingthe urging of the reciprocating probe.
 6. A method according to claim 1,wherein urging comprises urging the probe substantially perpendicular tothe interface.
 7. A method according to claim 1, further comprisingpositioning at least one structural member and spacer on an oppositesurface of the substrate and thereafter urging the reciprocating probethrough the interface defined between the opposite surface of thesubstrate and the at least one structural member such that a structuralmember is capable of being friction stir welded to the opposite surfaceof the substrate.
 8. A method according to claim 1, further comprisingpositioning a shield carried by the probe adjacent to the structuralmember and spacer, the shield capable of securing the spacer within theaperture during the urging of the probe.
 9. A method according to claim1, further comprising machining the preform to form a structuralassembly having a predetermined configuration.